Enable suggested extensions (rubocop-rake)

In !119 (merged), I set SuggestExtensions: false to avoid making the upgrade MR too big. However, we should probably take rubocop-rake and allow suggestions in future.

Tip: Based on detected gems, the following RuboCop extension libraries might be helpful:
  * rubocop-rake (https://rubygems.org/gems/rubocop-rake)

You can opt out of this message by adding the following to your config (see https://docs.rubocop.org/rubocop/extensions.html#extension-suggestions for more options):
  AllCops:
    SuggestExtensions: false